Hydrogen peroxide is an oxidant utilised in several applications such as chemical epoxidation processes, waste-water purification, sterilisation of industrial cooling circuits, treatment of electronic integrated circuits, and whitening in textile and paper industries. In these applications, hydrogen peroxide is particularly advantageous as it does not generate any noxious residues, the only final product consisting of water.
The manufacturing process currently employed is known as an anthraquinone process, wherein ethylanthraquinone (or derivatives thereof) dissolved in a suitable organic solvent is sequentially reduced and oxidised, with final generation of a 20% hydrogen peroxide primary solution, subsequently concentrated by distillation. This process is suitable for plants of large capacity, typically 50,000 t/yr or more, the final product being distributed to the different users in tankers or steel containers as 35-50% hydrogen peroxide solution.
Users must, therefore, accept all the inconveniences associated with the decay of stored hydrogen peroxide concentration and with the handling of the tanks. To appreciate the importance of the latter, it will suffice to remind that the sterilisation treatment of an industrial cooling circuit characterised by a 50,000 m3/h flow-rate and by an injection of as little as 2 parts per million hydrogen peroxide requires about 2500 m3/yr of 35% solution.
Furthermore, some applications require hydrogen peroxide free of noxious (in the case of waste-water purification) or adsorbable (in the case of integrated circuit treatment) organic impurities. Under the latter standpoint, the use of hydrogen peroxide obtained by means of the anthraquinone process is problematic, since the commercial product may contain organic substances up to 100 ppm as well as non-negligible traces of metals, wherein the organic substances and the metal are respectively released by the process solvent and by the plant machinery which may be subject to some corrosion. The known methods for treating hydrogen peroxide concentrated solutions do not seem to be capable of decreasing the impurities to the levels required by the most critical applications.
It is clear, then, that a process suited to the localised production of highly pure hydrogen peroxide would be received with favour by at least a portion of the users. Processes of this kind are known from the technical literature. Both purely chemical processes based on the direct combination of oxygen and hydrogen on suitable catalysts in the presence of adequate additives at controlled temperature and pressure conditions and electrochemical processes have been proposed. For instance, electrochemical processes capable of producing dissolved hydrogen peroxide at a concentration of 1-3% in alkaline electrolytes, typically 5-10% sodium hydroxide, have been reported.
This process presents an interesting faradic efficiency (expressed as percentage of electric current effectively used for generating hydrogen peroxide), but is also affected by two important drawbacks, one being the presence of an alkaline electrolyte which narrows the number of applications of product hydrogen peroxide. For example, in industrial cooling plants, it is largely preferred the addition of sterilising agents not altering the pH of circulating water, while in the cleaning treatment of integrated circuits the agents employed, which must be easily decomposable without forming secondary products, must not contain additional components except at minimum levels, in the order of magnitude of parts per billion at most.
A second drawback is given by the criticality of operation of gas-diffusion electrodes used for the conversion of oxygen to hydrogen peroxide, when these necessarily porous electrodes are in contact with a liquid phase, in this case the alkaline solution. Since industrial cells are tall, the consequent hydraulic head determines a flooding of the gas-diffusion electrode in its lower portion, which practically stops functioning properly. For this reason, the design of electrolysis cells equipped with gas-diffusion electrodes entails a limitation in the height and consequently in the active surface, lessening the productive capacity to such an extent that an industrial application proves not viable.
Some attempts directed to overcome this inconvenience are disclosed in the technical literature, but for the time being they have not been developed enough to make them suitable for a commercial use.
One way to radically solve the problem of internal flooding of oxygen-diffusion electrodes has been proposed in which a cell subdivided by an ionomer membrane into two compartments, cathodic and anodic, respectively fed with oxygen and with water. The ionomer membrane is provided with two electrodes, cathode and anode, in the form of catalytic porous films, respectively, for the reduction of oxygen to hydrogen peroxide on the cathode side and for evolving oxygen from water on the anode side. The oxygen evolution reaction releases protons which migrate in a hydrated form across the ionomer membrane and react with oxygen in the cathode porous film generating hydrogen peroxide. The membrane isolates the cathode porous film from the hydraulic head established by the water present in the anodic compartment. Flooding of the porous cathode is hence no longer possible, so that the electrolysis cell may be designed of suitable height for industrial applications. The problem with this type of process is given by the faradic efficiency of hydrogen peroxide production which is around 3-3.5%, with final concentrations of 1 to 1.5%. Such a modest result probably derives from the lack of substantial dilution of generated hydrogen peroxide which is conversely a peculiar feature of the alkaline-type process. In fact, in these processes hydrogen peroxide is diluted by the proton hydration water alone, since water diffusion across currently employed ionomer membranes is not significant. In a simplified calculation, assuming a faradic efficiency of 50% and four water molecules constituting the proton hydration shell, a theoretical hydrogen peroxide concentration around 10% is obtained. At this concentration level, hydrogen peroxide is probably affected by a substantial decomposition rate inside the cathode film, certainly accelerated by traces of even minimal amounts of some elements, in particular transition elements and compounds thereof.